Structures and functions of enzymes involved in primary and secondary metabolite biosynthesis mediated by amino-group carrier protein.

Thermus thermophilus, an extremely thermophilic bacterium, synthesizes lysine through a novel type of 2-aminoadipate (AAA) pathway. The pathway starts from the synthesis of homocitrate using 2-oxoglutarate and acetyl-CoA. Homocitrate is converted to 2-oxoadipate by sequential reactions similar to the corresponding reactions in the tricarboxylic acid cycle. AAA formed by the transamination of 2-oxoadipate is loaded on amino-group carrier protein (AmCP) via isopeptide bond linkage. AAA is successively converted to lysine on AmCP by several reactions, and lysine is finally released from the carrier protein. A similar AmCP-mediated system is found in lysine and ornithine/arginine biosyntheses of archaea. Furthermore, the AmCP-mediated system is diversified in secondary metabolite biosynthesis with some modifications in various bacteria, including Streptomyces. This review shares the AmCP-mediated biosynthesis system and its contribution to expanding the structural diversity of natural compounds, along with the evolutionary aspects of lysine and arginine biosyntheses.